In My employee map i have my salary set to decimal(25,4).while querying the above, NH is generating the following

cast(sum(employee.Salary) as DECIMAL(19,5)) as col

As I have not given any particular map to the result , nhibernate is casting my result.sal to decimal(19,5) by default.However my data is containing more than 19 digits and this is resulting in an arithmetic overflow exception. Is there a way to avoid this,like asking NH to consider casting my result as (25,4) instead?